fix(IBA): demosaic - fix roi channels

[antond-weta]

Description

Fixes a bug in demosaicing where the code ignores the ROI channels.
The old behaviour was to always decode into the first 3 channels.

Tests

None of the current tests use preallocated output buffers. Should we have some? The change is fairly trivial though.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[lgritz]

LGTM